The present invention relates to the formation of semiconductor devices. More specifically, the present invention relates to removal of photoresist.
During semiconductor wafer processing, features of the semiconductor device are defined in the wafer using well-known patterning and etching processes. Photoresist (PR) is used to protect some areas of the wafer from etch chemistries so as to define the features. Photoresist is also used as ion implantation masks for implanting a dopant into a silicon substrate. Conventionally, photoresist masks are removed by a “wet” process in back end of line (BEOL) processing, for example, after metallization, while a “dry” process is used in front end of line (FEOL) processing, for example, after ion implantation that characterizes devices. However, dry stripping may be used in the BEOL processing, and wet stripping may be used in the FEOL processing. Dry photoresist strip may be conducted either in a downstream environment or in a direct low bias-potential plasma. In FEOL processing, active areas (Si or SiGe), doped or undoped, are subjected to various levels of implants, followed by photoresist strip and post strip cleans.
Effects on active area from dry or wet processes can be detrimental from a material or dopant loss prospective. Material loss can occur as the active area is subjected to repetitive strip-and-cleans or etch processes. Material loss is generally defined as the conversion of active species such as silicon or dopants to their inactive compounds such as oxides. As material loss increases, various device characteristics, such as drive current, leakage, resistivity, and short channel effects, also change. Device sensitivity to material loss increases even further as device geometries decrease 45 nm and beyond, where junctions are shallower and more lightly doped by high-flux, low-energy ion implantation. Active area characteristics are a precision-engineered part of any device for optimum performance, and therefore, material loss due to FEOL processing—such as post ion implant strip (PIIS), may be detrimental to device performance. Furthermore, the ions implanted in the photoresist chemically modify the near-surface regions, causing decomposition, cross-linking, etc., of the photoresist. Such chemically modified regions form a hard crust, where the polymer may become graphitic in nature. Such a hard crust is typically formed in the upper resist region and at exposed sidewalls.
In order to avoid material loss due to ion bombardment in dry strip processes, improved wet processes with a hot sulfuric acid have also been used. However, typical wet photoresist strip chemistries, such as hot Piranha solutions (H2SO4:H2O2), do not work well when such crusts exist. In order to strip such crusts, aggressive plasma strip chemistries are being used, which typically include highly oxidizing radicals. To enhance the crust strip, the wafer may be heated, and fluorine (F)-containing species may be added to the plasma. However, use of such harsh chemicals (i.e., reduction or fluorination) and/or oxidation leads to Si material loss as well as dopants and Ge, which impacts device performance.